The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
When a vehicle is coast driving, an engine is controlled in a fuel cut state in which fuel injection is not performed and an automatic transmission is controlled so that the engine is not stalled through the deceleration direct connection of a damper clutch.
However, in this case, the engine serves as a load of the vehicle, and as a result, a coasting driving distance is reduced and when the engine is kept in the fuel cut state for a long period of time, a temperature of a catalyst is lowered and oxygen is adsorbed to the catalyst. Accordingly, there is an issue that the fuel must be injected richly in order to raise the catalyst temperature at an initial stage of re-acceleration of the engine, which generates NOx.
A coasting mode is a control method that shifts the automatic transmission to an N-stage state through shifting hydraulic control during the coast driving.
In the coasting mode, the engine and the vehicle are separated from each other because the transmission is in the N-stage state and the coast driving distance increases. Therefore, the number of times of stepping an accelerator may be reduced when the vehicle is driven on the same distance, thereby improving fuel efficiency of the vehicle.
In addition, since a frequency of entry of the engine into the fuel cut is reduced and the temperature of the catalyst is not lowered, NOx can be reduced when the engine is re-accelerated.
When an accelerator pedal is operated in an entering state of the coasting mode, the engine and the transmission connect to a D-stage again in order to accelerate the vehicle.
Generally, since an increase of engine torque speed is greater than a rise of transmission oil pressure speed, a D-stage connection hydraulic control signal is entered to the automatic transmission so that the torque-to-shift connection oil pressure shortage does not occur. At the same time, an engine torque limitation (ETL) signal is entered to the engine.
However, when a shift ends, a torque limit is released. As a result, the engine torque rises rapidly and an engine-turbine reversal in which the engine RPM is changed to a state that the engine RPM is greater than the turbine RPM may occur.
The above information disclosed in this section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.